Is Java “pass-by-reference” or “pass-by-value”?

Are you confused about whether Java uses pass-by-value or pass-by-reference? You’re not alone! This is a common question for Java developers. Let’s break down this concept with clear explanations, analogies, and code examples.

The Short Answer: Java is Always Pass-by-Value

Yes, you read that right. Java always uses pass-by-value. However, the key to understanding this lies in what is being passed:

• For primitive types (like int, boolean, double), the actual value is copied and passed.
• For objects, the value that is passed is a reference (a pointer) to the object’s location in memory.

Analogy: The Realtor and the Tiny House

Imagine a realtor, Bob, who manages listings. Bob has a listing (tinyHouseAt1234Main) for a blue tiny house. This listing includes details and a viewTally.

• Pass-by-Value: When someone asks Bob about the house, he provides a copy of the listing details, not the actual house or its address.
• The Tricky Part: Now, imagine the person has access to a service that can modify the actual house (repaint it, change the landscaping). Even though they have a copy of the listing, their actions change the original house.

Code Example

class Point {
    int x;
    int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

public class PassByValueExample {

    public static void tricky(Point pnt1, Point pnt2) {
        System.out.println("X: " + pnt1.x + " Y: " + pnt1.y);
        System.out.println("X: " + pnt2.x + " Y: " + pnt2.y);

        pnt1.x = 100;
        pnt1.y = 100;

        System.out.println("X: " + pnt1.x + " Y: " + pnt1.y);

        pnt1 = new Point(0, 0);
    }

    public static void main(String[] args) {
        Point pnt1 = new Point(0, 0);
        Point pnt2 = new Point(0, 0);

        System.out.println("X: " + pnt1.x + " Y: " + pnt1.y);
        System.out.println("X: " + pnt2.x + " Y: " + pnt2.y);

        tricky(pnt1, pnt2);

        System.out.println("X: " + pnt1.x + " Y: " + pnt1.y);
    }
}

• pnt1 and pnt2: These are object references holding memory addresses.
• tricky Method: The values of pnt1 and pnt2 (the memory addresses) are copied. Modifying pnt1.x and pnt1.y changes the original Point object. However, pnt1 = new Point(0, 0) creates a new Point object, and only the local pnt1 reference is updated.

Key Concepts

• Pass-by-Value: Java always passes arguments by value.
• Object References: For objects, the value passed is the reference.
• Modifying Objects: Changes to the object’s state through the reference are reflected in the original object.
• Reassigning References: Reassigning the reference (e.g., pnt1 = new Point(0, 0)) does not change the original reference.

Why This Matters

Understanding pass-by-value with object references is crucial for:

• Predicting how methods will affect your objects.
• Avoiding unexpected side effects in your code.
• Writing correct and efficient Java programs.

In Simple Terms: The Pet Dog Analogy

Imagine you leave your pet dog, Jimmy, with a vet.

• If the vet modifies Jimmy (cuts his tail), the original dog is changed.
• If the vet clones Jimmy and modifies the clone, your original dog remains the same.

Key Differences: Pass-by-Value vs. Pass-by-Reference

Conclusion

Java is always pass-by-value. When working with objects, remember that you’re passing a copy of the reference, which allows you to modify the original object, but not reassign the original reference itself.

Citations:
[1] https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value/58704042
[2] https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value/58704042
[3] https://www.digitalocean.com/community/tutorials/java-is-pass-by-value-and-not-pass-by-reference
[4] https://www.cs.virginia.edu/~jh2jf/courses/cs2110/java-pass-by-value.html
[5] https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value/29133165
[6] https://www.softwaretestinghelp.com/java/java-pass-by-reference-and-pass-by-value/
[7] https://www.reddit.com/r/java/comments/oxc75/can_someone_explain_pass_by_value_in_java_please/
[8] https://www.upgrad.com/tutorials/software-engineering/java-tutorial/pass-by-value-and-call-by-reference-in-java/
[9] https://www.baeldung.com/java-pass-by-value-or-pass-by-reference
[10] https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value/41901960
[11] https://www.edureka.co/blog/pass-by-value-pass-reference-java/
[12] https://news.ycombinator.com/item?id=9245043
[13] https://sentry.io/answers/java-pass-by-reference-or-value/
[14] https://www.javatpoint.com/java-pass-by-value
[15] https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value/58704042